Title: Experimental investigation of the atmospheric chemistry of aromatic hydrocarbons and long-chain alkanes

Principal Investigator / Author(s): Atkinson, Roger

Contractor: UC Riverside

Contract Number: A032-067

Two aspects of the atmospheric chemistry of volatile organic compounds have been addressed in this experimental program, and these concern the reactions of the hydroxycyclohexadienyl-type radicals (OH-aromatic adducts) formed from OH radical addition to the aromatic ring of the aromatic hydrocarbons under atmospheric conditions and the atmospheric reactions of the alkoxy radicals formed after OH radical reaction with the alkanes. Product studies of the OH radical-initiated reactions of toluene, o-xylene and 1,2,3-trimethylbenzene have been carried out in the absence of NOx and in the presence of varying Concentrations of NO2. The 2,3-butanedione formation yields from the o-xylene reaction show that the OH-o-xylene adduct reacts with both O2 and NO2. The rate constant ratio for reaction of the OH-o-xylene adduct with NO2, versus reaction with O2 was determined to be kno2/ ko2 = 1.3 x l05 at 298 K, showing that the OH-o-xylene adduct reacts with O2 in the atmosphere. However, since several product studies of the OH radical reactions with aromatic hydrocarbons have been carried out at part-per-million levels of NO2, care must be taken in using these laboratory product data for atmospheric purposes. A series of product studies of the OH radical-initiated reactions of selected alkanes, ketones and alcohols have been carried out to obtain further insights into alkoxy radical isomerization. Product studies with gas chromatographic analyses of the OH radical reaction with 4-methyl-2-pentanone, 2,6-dimethyl-4-heptanone, 2,4-dimethyl-2-pentanol and 3,5-dimethyl-3-hexanol, in the presence of NOx, have provided unambiguous evidence for alkoxy radical isomerization and these studies and that of n-pentane, again using gas chromatography for product analyses, have provided rate constant ratios for the isomerization reaction versus alkoxy radical decomposition and reaction with O2. Preliminary experiments with in situ atmospheric pressure ionization mass spectrometry are consistent with the formation of the expected 8-hydroxcarbonyls formed after the alkoxy radical isomerizations, with the apparent importance of isomerization increasing with the carbon number for the n-alkane series n-butane through n-octane. The experimental data obtained in this Contract provide new information concerning the atmospheric chemistry of aromatic hydrocarbons and alkanes which will allow future detailed chemical mechanisms for the formation of photochemical air pollution to be placed on a firmer scientific basis than is presently the case.

For questions regarding this research project, including available data and progress status, contact:
Heather Choi
at (916) 322-3893